The present invention relates to a servo circuit for a capstan motor used in an apparatus for recording/reproducing digital data or the like, and more particularly to a servo circuit capable of obtaining a perfect continuation of recorded data at a joint section of a recording medium from which section new data were recorded (append-write) after already recorded data.
Rotary head type digital audio tape recorders (R-DAT) are available in markets as audio equipment of high sound quality. Studies have been made extensively for using R-DATs as external storage devices for computers. If an R-DAT is used as an external device, the storage capacity per one roll of a recording tape becomes as large as 1 giga (G) bytes.
If a conventional R-DAT itself is used as an external storage device, there arises a problem that the data recorded at a joint section during append-write cannot be correctly read (reproduction error). The reason for this is that track pitches before and after the joint section during append-write are not equal, namely, tape speeds are not equal.
Even if there are some musical signal data which could not be read correctly, the data can be modified through error correction or interpolation schemes to the extent that human acoustic sense does not recognize error modification. However, such error modification is not allowed for the case of computers and the like.
Append-write operation using a conventional R-DAT will be described below with reference to FIGS. 1 and 2. FIG. 1 shows a system arrangement of a conventional servo circuit in block, and FIG. 2 is a diagram used for explaining the operation of the servo circuit.
Referring to FIG. a rotary drum having magnetic heads (simply called heads hereinafter) 2a and 2b, is controlled to rotate at a constant velocity by a dedicated servo circuit (not shown). A capstan motor 3 is controlled during a playback mode by a sum of signals from a velocity servo system and tracking system, and during a recording mode by a sum of signals from the velocity servo system and phase servo system.
For the capstan servo during the playback mode, a tracking signal (position detecting signal) recorded on a magnetic tape (hereinafter called a tape) 4 is read by the heads 2a and 2b and amplified by an amplifier 5. The tracking signal is then supplied to a tracking position detecting circuit 6 and transformed into a tracking error signal (position error signal) S.sub.a. A waveform of the signal S.sub.a is shown in FIG. 2(d) (tracking detecting system).
The tracking position detecting circuit 6 detects a relative position of the tape 4 and heads 2a and 2b in the running direction of the tape 4.
During the recording mode, the tracking signal is generated by a signal processing circuit 10 and recorded on the tape 4.
In the velocity servo system, an FG pulse signal S.sub.d obtained by a frequency generator (FG) which rotates together with a capstan motor shaft 7, is supplied to a velocity detecting circuit 9 which generates a velocity error signal.
The velocity error signal and tracking error signal S.sub.a via a switch 12 are added together by an adder 13. In accordance with the addition signal, a control amplifier 14 controls a drive voltage to be applied to the capstan motor 3 to thereby maintain the tape transporting speed constant. In this case, the velocity error signal controls the high velocity region, and the tracking error signal controls the low velocity region. In addition, the velocity error signal has the role of a coarse control, and the tracking error signal has the role of a fine control.
Next, the capstan servo during the recording mode will be described. A clock signal S.sub.e of high precision obtained from a clock generator 15 constructed of a crystal oscillator and a frequency divider, as well as the FG pulse S.sub.d, is applied to a phase comparator 16 which generates a phase error signal S.sub.b (phase servo system). The signal S.sub.b is supplied via the switch 12 to the adder 13, and the velocity error signal is directly supplied to the adder 13, to thereby control a drive voltage for the capstan motor 3.
The switches and 12 and the signal processing circuit 10 are controlled by signal/switch control circuit 17. The switch 12 is maintained to be turned off by the control circuit 17 until a signal is detected from the tape.
Next, there will be described operation during the append-write (new data are recorded immediately after the area where data have been already recorded on the tape 4).
If the stop mode is to be changed to the recording mode, the recording mode is not directly enabled, but the playback mode is first executed as shown in FIG. 2 (at time t.sub.0). Even after the tracking signal to be reproduced becomes stable, that is, after the tracking error signal S.sub.a becomes stable (after time t.sub.l), the frequencies of the clock signal S.sub.e and FG pulse S.sub.d are not correctly coincident with each other because of errors caused by extension/contraction of the tape 4 or the like. Accordingly, even after the time t.sub.1, the phase error signal S.sub.b may fluctuate as shown by the waveform in FIG. 2(e).
If the control circuit 17 judges that the tracking error signal S.sub.a has been stabilized sufficiently, the operation switches to the recording mode (time t.sub.2) so that the tracking signal and data signal supplied from the signal processing circuit star being recorded. At this time, the switches 11 and 12 are also turned to the recording mode side. At the transition period (at data junction section) from the playback mode to the recording mode, the following problems occur.
At the time t.sub.2 when the recording mode starts, the heads 2a and 2b are tracing the correct position and the tape 4 is running at a correct speed. However, at the same time t.sub.2, the phases of the clock signal S.sub.e and FG pulse S.sub.d are not equal in most cases. Accordingly, an output signal S.sub.c from the switch 12 (the output signal S.sub.c changes to the phase error signal S.sub.b after the time t.sub.2) is disturbed greatly in the negative direction (or positive direction) as shown in FIG. 2(f). The servo circuit responds to the disturbance and greatly changes the drive voltage to the capstan motor so as to quickly follow the capstan phase at that time. As a result, the tape speed is disturbed as shown in FIG. 2(g). Signals recorded at the junction where the tape speed fluctuate cannot be read correctly during the playback mode. A conventional servo circuit therefore cannot realize seamless append-write (recording without a joint).
The tape speed becomes stable after time t.sub.3 because the phase of the clock signal S.sub.e and FG pulse S.sub.d are made coincident by the servo circuit.
The above problem may be solved if the phase of the clock signal S.sub.e is made coincident with that of the FG pulse S.sub.d immediately before the switching from the playback mode to the recording mode. However, it is difficult for this method to be adopted to the conventional IC (integrated circuit) servo circuit in many cases. The reason for this is that a conventional servo IC is integrally built with the tracking position detecting circuit 6, velocity detecting circuit 9, switch 12, clock generator 15, phase comparator 16, and a part of the control circuit 17.
The operation of recording data on a new tape by the servo circuit will be described with reference to FIG. 3.
As shown in FIG. 3(c), the switch 12 is maintained turned off before the time t.sub.2. While the switch 12 is maintained turned off, the output from the switch 12 is fixed at the center level of the output ranges from the tracking position detecting circuit 6 and phase comparator 16 (the level with no error of the tracking error signal S.sub.a and the level with phase difference 0 degree of the phase error signal S.sub.b). Accordingly, the capstan motor 3 is servo-controlled by only the velocity servo system until the time t.sub.2.
As described previously, the phase of the FG pulse S.sub.d is not generally coincident with that of the clock signal S.sub.e at the time t.sub.2, and the level of the phase error signal S.sub.b is not coincident with the center level of the output range of the phase comparator 16. Accordingly, when the switch 12 is turned to the recording mode side at the time t.sub.2 and the phase servo system is incorporated, this servo circuit operates to correct the phase quickly, thereby posing the problem of disturbance of the tape speed.